1. Field of the Invention
The invention pertains to distortion sensing apparatus for use with telecommunications links.
2. The Prior Art
In the case of transmission of digital signals by means of radio, and in the specific case of adaptive procedures incorporated into the design of send-receive stations, such as mobile receiving stations, there exists a need to monitor the receivability of these signals, in order to differentiate them from interferences and faulty signals. As soon as a signal is faulty over a certain degree, the danger exists that falsifications or losses of the message content will occur. The degree of the interference of signals is generally expressed as signal/noise ratio S/N, which corresponds to the energy relationship of the wanted signal P.sub.s to the unwanted signal P.sub.N. EQU S/N=10.multidot.1g(P.sub.s /P.sub.N)dB (1)
In the case of transmission of digital signals, or digital signallings, the effective signal/noise ratio expresses itself in the so-called "phase jitter" of the digital signal. The jitter magnitude J is a measure of the mean percental dispersion or variation of zero passages of the digital signal in the base band, with reference to the undistorted length of an individual bit.
The receivability of incoming signals on a transmission link such as a radio link may be ascertained by making use of a field strength measurement. The field strength alone indeed is not a complete measure of signal quality, as interferences which overlay the actual desired signal can produce an apparently sufficient signal field strength. As the German OS No. 2,659,635 shows, it is necessary to monitor the receivability of an incoming signal not only by means of a field strength measurement, but also or exclusively by means of a phase jitter measurement.
A phase jitter measurement can be carried out by means of a time window, by means of which the regenerated digital signal on the receiver side is observed and the mean percental dispersion of the zero passage of the pulse edges is determined. Such a measurement, however, presumes that the regenerator on the receiver side is adjusted, by means of a reference frequency, to the bit rate of the incoming signal with sufficient accuracy. This reference frequency is produced by means of a local clock pulse oscillator whose output can be controlled by means of a phase regulating loop. As soon as the synchronization of the phase regulating loop has occurred, both the beginning as well as the end of each received signal bit are known at the receiving location. Such an arrangement is specified in the German No. AS 2,047,944. In general, the band width of the phase regulating loop is selected small enough that an interference free reference signal is delivered by the clock pulse oscillator. This reference signal defines the beginning and the end of the undistorted receiver-side bits even in the case of relatively low values of S/N by means of an integrating mean value formation of the phase regulating loop.
The above described form of phase jitter measurement of the incoming signal is disadvantageous because it presumes that the phase regulating loop of the regenerator has been synchronized. Therefore, the measurement of the quality of the signal which is received can only be made after synchronization has taken place. Also disadvantageous is that in the case of a decreasing signal/noise ratio S/N, the necessary interference free reference signal can only be obtained by correspondingly reducing the band width of the phase regulating loop. This, however, leads to longer synchronizing times. Thus, the lost time before the quality measurement may be taken increases with a decreasing signal-to-noise ratio. If no synchronization can occur because the signal-to-noise ratio S/N is too low, or the interference is too high, then the non-receivability of the incoming signal can be determined only after a corresponding waiting time, which corresponds to the maximum synchronizing time.
The mentioned disadvantages are particularly aggravated in the case of fast frequency changeable and adaptive radio systems, which function either with frequency jump methods, automatic channel seeking or other frequency variation devices. The minimum times which are necessary in the case of such radio systems for the adaptation processes to take place are significantly enlarged because of the phase jitter measurement. In the case of distorted radio channels, large intervals of time can pass before a determination can be made that the connection is unsuitable.
Thus, there is an unfilled need for an apparatus and method to quickly measure phase jitter which does not require a synchronized regenerator in the receiver unit.